Generally, bicycle speed control devices operate the control wire forwardly or backwardly to actuate the derailleur so that a chain guide thereof shifts a driving chain to one desired sprocket of a multistage sprocket assembly provided at a crank shaft or a rear hub of the bicycle. Most speed control devices employ a positioning mechanism provided with a positioning member having a plurality of engaging portions for setting the speed change stages and at least one engaging member engageable with one of the engaging portions so that a lever is operated to engage the engaging member with one engaging portion to thereby change the speed change stage and position the lever body at the desired speed change stage.
In a case where the chain is shifted to one sprocket of the multistage sprocket assembly, the chain, even when somewhat shifted from the proper engaging portion, does not make contact at its link plate with the sprocket, because each sprocket has a stable region of a predetermined width by which the generation of noise is prevented. However, the chain, when shifted to the desired sprocket from the position out of the stable region, makes contact with the sprocket or a chain guide to produce noises.
In other words, the stable region is different in width due to a configuration of the chain, the shape of teeth at the sprocket and a combination of sprockets at the multi-stage sprocket assembly, but when the chain is positioned within the stable region, there is no risk of generating noises.
With respect to bicycle speed control devices which use the control wire to operate the derailleur and set the speed change stage of the lever body by use of the positioning mechanism, the positioning mechanism sets the speed change stage at the same position, when the wire is operated by a lever body forwardly or backwardly. Moreover, the control wire varies in its effective stroke, such that the chain guide at the derailleur is placed in a different position when the control wire is operated forwardly or backwardly by the lever body.
It is not problematical that the chain guide is positioned within the stable region during both the forward operation and backward operation of the control wire, but a problem is created in that when the chain guide is set to be positioned within the stable region upon either one of the forward and backward operations, the chainguide is shifted out of the stable region at the other operation.